If I am reading correctly, you stated the smell was gone after the antibiotics?
There is no signs of yeast?
Is there any scabs on his skin?
How often are you bathing him? Just an idea, but if you bath tooooo much, his skin will get dried out and this, in return, his body will return the fight with more "natural" bully oils, which unfortunately to us, does not smell all that pleasant.
Have you asked your vet about this yet?